Polychlorinated biphenyls, or "PCB's", were long used as dielectric fluids in electrical equipment because these materials have excellent heat stability, are non-flammable in nature, have low volatility and good viscosity characteristics at operating temperatures. Because of their environmental persistence, however, continued manufacture, import or use in the United States was banned under the Toxic Substance Control Act of 1976, and the U.S. Environmental Protection Agency was directed to promulgate rules and regulations for their removal from the economy.
As of July 1, 1979, EPA regulations defined as "PCB-contaminated" any material containing more than 50 ppm of a mono, di or polychlorinated biphenyl. The regulations permit the disposal of PCB-contaminated materials by either incineration in an approved manner or in an approved landfill, but such procedures have rarely proven acceptable to community neighbors.
Since considerable fractions of the transformer oils and heat exchange oil now in service are PCB-contaminated, the problem of disposing of PCB-contaminated hydrocarbon oils in an effective manner presents a serious challenge. Various techniques for degrading a polyhalogenated aromatic hydrocarbons in such oils have been proposed. Most techniques are too lengthy and/or complex to provide a practical solution to the problem of PCB-contaminated oil. However, processes disclosed by Brunelle in U.S. Pat. Nos. 4,353,739, 4,351,718 and 4,410,422 provide rapid and effective degradation of PCB's in such oils. These processes utilize alkali metal hydroxides in combination with polyalkylene glycols and/or monoalkyl ethers of polyethylene glycol as a reagent to remove PCB's. These patents are assigned to the same assignee as the present invention and are incorporated herein by reference.
When removing PCB's from a solution on a large scale, such as in a commercial process, it is desirable to maximize the rate of reaction. One method for enhancing the rate of reaction in the Brunelle processes is to increase the surface area of the alkali metal hydroxide by utilizing smaller solids and providing a better suspension of solids. The KOH obtained commercially is in the form of pellets and/or flakes and must be crushed prior to use in the Brunelle process. The crushed particles provide a higher surface area and a better suspension. A corresponding increase in the reaction rate is obtained with these crushed particles.
Although crushing of the KOH pellets and/or flakes provides suitable results, there is room for improvement. Due to the hygroscopic nature of KOH, the crushing and handling of the KOH powder must be carried out in an inert atmosphere such as nitrogen. This complicates the crushing procedure. Furthermore, due to the limitation on grinding equipment, a wide particle size distribution is obtained for the KOH powder produced. This particle size distribution affects mixing procedures and mass transfer procedures in the reaction vessel. The system utilized must accept wide variances in solid size when mixing or transferring the reactor contents or the alkali metal hydroxide will form a sticky viscous mass on the equipment surfaces.
The present invention provides an improved method for removing polyhalogenated aromatic hydrocarbons from inert, organic solvent solutions which maximizes the surface area of the alkali metal hydroxide by forming fine solids without the use or complications of a grinding step. The solids obtained are smaller than those generated from conventional grinding processes and provide a more uniform suspension in the reaction medium. This uniform suspension enhances the reaction rate and permits the process to be adapted to continuous operation.